Light-scattering translucent products of transparent plastics with various light-scattering additives and mouldings produced therefrom are known.
US 2004/0066645 A1 describes in general light-scattering materials which comprise 0.2 to 5% of light-scattering particles and which have a light transmission of greater than 70% and a haze of at least 10%.
The scattering additive has an average diameter of from 3 to 10 μm.
JP 07-090167 describes a light-scattering plastics which comprises 1 to 10% of particles which have a refractive index of less than 1.5 and a particle size of from 1 to 50 μm and 90 to 99% of an aromatic polycarbonate, the particles substantially being insoluble in the aromatic polycarbonate. Acrylate, polystyrene, glass, titanium dioxide or calcium carbonate particles are employed as scattering additives. LCD are mentioned as a use.
EP 0 269 324 B 1 describes a scattering additive composition and light-scattering thermoplastics polymer compositions comprising 0.1 to 10% of scattering additive.
In EP 0634 445 B1, Paraloid EXL 5137 is employed as a scattering additive in combination with inorganic particles in polycarbonate. 0.001 to 0.3% of these particles (e.g. titanium dioxide) contributing towards an improved resistance to ageing and therefore colour stability. This is particularly advantageous if compounds having high scattering agents contents (>2%) are exposed to elevated use temperatures (e.g. 140° C.) over a relatively long period of time (>500 hours).
WO 2004/090587 describes diffuser sheets having a thickness of 2 mm for use in LCD, which comprise 0.2 to 10% of scattering additive and have a gloss of 20 to 70% (measured in accordance with JIS K 7105) at least on one side. Coextruded sheets in which the coextruded layer is 20 to 200 μm thick are furthermore described. The base and coextruded layer comprise scattering additives. Crosslinked silicones, acrylates or talc are compounded in as scattering additives which have a particle diameter of from 5 to 30 μm.
The diffuser films known from the prior art have an unsatisfactory brightness, in particular in conjunction with the set of films conventionally used in a so-called backlight unit. To evaluate the suitability of the light-scattering sheets for so-called backlight units for LCD flat screens, the brightness of the total system must be considered.
In principle, a backlight unit (direct light system) has the structure described in the following. As a rule, it comprises a housing in which a varying number, depending on the size of the backlight unit, of fluorescent tubes, so-called CCFL (cold cathode fluorescent lamp), are arranged. The inside of the housing is equipped with a light-reflecting surface. The diffuser sheet, which has a thickness of from 1 to 3 mm, preferably a thickness of 2 mm, is positioned on this illumination system. On the diffuser sheet is a set of films which may have the following functions: light-scattering (diffuser films), circular polarizers, focusing of the light in the forwards direction by so-called BEF (brightness enhancing film) and linear polarizers. In this context, the linearly polarizing film is positioned directly under the LCD display on top.
Light-scattering plastics compositions in optical uses conventionally comprise inorganic or organic particles having a diameter of from 1 to 50 micrometers, and in some cases even up to 120 μm, i.e. they contain scattering centres which are responsible both for the diffusive and for the focusing properties.
In principle all acrylates, in particular core-shell acrylates, which are thermally stable up to at least 300° C., in order not to be destroyed at the processing temperatures of the transparent plastics, preferably polycarbonate, may be employed as transparent scattering pigments. Furthermore, the pigments should have no functionalities which lead to a degradation of the polymer chain of the polycarbonate. Thus e.g. Paraloid® from Röhm & Haas and Techpolymer® from Sekisui are particularly suitable for use for pigmenting transparent plastics. A large number of various types are available from these product lines. Core-shell acrylates from the Paraloid series are preferably employed.
The object of the present invention was to provide films which have improved properties compared with currently used films, in particular improved brightness with simultaneously high light-scattering, so that they are particularly suitable for use for backlight units.